The present invention relates to optical filters, and more particularly, to tunable Fabry-Perot optical resonators, filters and lasers constructed therefrom.
Tunable optical resonators are utilized in optical communication systems and in the construction of lasers. Optical filters and lasers based on Fabry-Perot resonators can be constructed using microelectromechanical machining (MEM) techniques, and hence, can, in principle, provide an economically attractive tunable filter or tunable laser. In such devices, a Fabry-Perot resonator cavity is formed between two mirrors. One of these mirrors is flat and located on a semiconductor substrate. The other mirror may be curved and is suspended on a number of micro-mechanical cantilevers. Application of a tuning voltage between the cantilevers and the substrate causes the suspended mirror to move towards the fixed mirror on the substrate, thereby reducing the spacing between the two mirrors of the Fabry-Perot resonator. Since the filter""s bandpass frequency is determined by the mirror spacing, a reduction in spacing between the two mirrors causes the resonant optical frequency of the cavity to increase. The shift in the resonant frequency enables the device to be used directly as a tunable bandpass filter. If an optically-pumped or electrically-pumped optical gain medium (active region) is placed in the cavity, the device becomes a tunable laser, with the lasing wavelength controlled by the resonant frequency of the Fabry-Perot cavity.
Prior art MEM Fabry-Perot filters exhibit a significant amount of noise, which limits the usefulness of these devices. The noise results from mechanical vibrations in the mirror connected to the cantilevers. This noise causes variations in the spacing between the mirrors, which in turn, causes the resonant frequency and amplitude of the light emitted from the filter to exhibit a corresponding noise spectrum. The noise broadening of the resonant frequency can double the width of the bandpass response of a filter and can substantially increase the linewidth of a laser constructed from such a filter, thereby making the filter or laser unsuitable for many applications.
The mechanical vibrations can be reduced by stiffening the cantilevers. The mechanical vibrations result from thermal noise in the movable mirror and its micro-mechanical cantilevers. Since the moveable mirror is in thermal equilibrium with the air around the mirror, the mirror vibrates with an amplitude determined by the air temperature and the mechanical properties of the mirror and support. This vibration causes mechanical fluctuations in the position of the movable mirror relative to the other mirror. These mechanical fluctuations, in turn, cause fluctuations in the resonant optical frequency of the Fabry-Perot cavity. The mechanical fluctuations are exacerbated by any mechanical resonance in the moveable mirror and its support cantilevers. Stiffening a cantilever increases the spring constant of the cantilever, and hence, reduces the amplitude of the fluctuations corresponding to any given equilibrium temperature.
Unfortunately, stiffening the cantilever also reduces the range over which the resonance frequency can be tuned. In general, there is a maximum deflection voltage that can be applied to the device. This voltage is usually determined by the driving circuitry and the voltage breakdown characteristics of the Fabry-Perot resonator structure. The maximum deflection determines the maximum range over which the frequency of the filter or laser can be tuned. Hence, stiffening the cantilever by increasing the thickness of the support arm also reduces the range over which the device can be tuned.
Broadly, it is the object of the present invention to provide an improved MEM Fabry-Perot resonator.
It is a further object of the present invention to provide a MEM Fabry-Perot resonator that has reduced noise while maintaining or improving the range over which the resonance frequency can be tuned.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a tunable optical cavity constructed from a fixed mirror and a movable mirror. The fixed mirror is attached to a substrate having a first electrically conducting surface. A support member having the moveable mirror supported thereon and having a second electrically conducting surface, is suspended above the substrate. A circuit applies an electrical potential between the first and second electrically conducting surfaces thereby adjusting the distance between the fixed and movable mirrors. The fixed mirror and the moveable mirror are positioned such that the mirrors form the opposite ends of the optical cavity. The distance between the fixed mirror and the moveable mirror is a function of the applied electrical potential. The thermally induced vibrations are reduced by utilizing an electrical feedback circuit that measures the distance between the mirrors. The feedback circuit dynamically changes the potential between the substrate and the support member so as to reduce fluctuations in the cavity resonance frequency. The instantaneous cavity resonance frequency can be measured by comparing the cavity resonance frequency with a standard optical signal, or by using a circuit for measuring capacitative coupling between the support member and the substrate. The feedback circuit varies the potential between the substrate and the support member so as to reduce the fluctuations in said measured cavity resonance frequency or the capacitance. The optical cavity of the present invention can be utilized in constructing a tunable laser by including an active layer for amplifying light trapped in the cavity. In the case of a tunable laser, the thermally induced fluctuations can be reduced by including an interferometer or other frequency-selective device to determine the instantaneous wavelength of the light from the laser. An electrical feedback circuit varies the potential between the substrate and the support member so as to maintain the measured instantaneous wavelength at a specified value.